The NFL season kicked off at the annual Hall of Fame game with a moment of silence for the four people killed earlier this week by a shooter who was targeting league headquarters in New York.

The NFL held a moment of silence before Thursday's Hall of Fame game to honor the four people killed earlier this week by a shooter who was targeting league headquarters in New York.

The 27-year-old man who shot and killed four people at a Midtown office building on Monday carried a note in his pocket, police sources told ABC News.

The brain disease commonly known as CTE, caused by repeated head trauma, has prompted a long reckoning at all levels of football. The shooter at NFL headquarters believed he was suffering from it.

The shooter behind a deadly rampage inside an office building in Midtown Manhattan on Monday evening was apparently targeting NFL offices there, New York City Mayor Eric Adams said Tuesday morning.Law enforcement sources say investigators recovered a three-page rambling note from gunman Shane Tamura's wallet in which he referenced the brain disease CTE.